1. Field of the Invention
The present invention relates to electrical systems for motor vehicles.
2. Description of the Related Art
In city driving, a substantial amount of the fuel consumed by a motor vehicle is consumed when the vehicle is at rest or decelerating. In fact, some studies indicate that fuel economy on the "Federal Urban Drive Schedule" can be increased by up to 20% by turning off the vehicle's engine while the vehicle is stopped or decelerating. When the engine is off, vehicle electrical loads would be supplied from the vehicle's storage battery.
In a vehicle whose engine is turned off during such fuel-saving opportunities, the engine will be cranked more frequently than in other vehicles. Providing the very high-power (though relatively low-energy) electrical pulse to crank and start the engine is, however, very stressful on storage batteries. In fact, the ability of a storage battery to provide the high-power pulse begins to degrade after only a relatively few repetitions, although the overall energy capacity of the battery is not degraded. Thus, a system which would reduce the exposure of the vehicle's storage battery to such high-power engine cranking pulses will prove advantageous.
The addition of a capacitor to the vehicle's electrical system to provide the high-power engine cranking pulse has been proposed, for example, in U.S. Pat. No. 5,146,095, issued to Tsuchiya et al. This system, however, relies on relays to manage the charging and discharging of the capacitor. Other designs may be more cost-efficient and reliable.
In vehicles whose engine stops and starts frequently, for example to take advantages of fuel savings which can be realized, quick vehicle restarts are preferable to minimize annoyance to the driver of the vehicle. Such quick vehicle restarts can be facilitated by providing a particularly large amount of power to the engine cranking motor. A cranking motor operating at a higher voltage than the conventional 12 volts of a motor vehicle electrical system can provide higher power without correspondingly higher electrical currents, an advantageous situation. But, if a cranking motor operating at a higher voltage is used in a system which otherwise uses 12-volt electrical loads, a decision must be made as to the voltage at which electrical energy will be generated and stored in the system. Generating and storing energy at 12 volts will require a voltage up-converter to provide the higher voltage for the engine cranking motor. Using an up-converter to convert a relatively lower voltage to a relatively higher voltage for supply to a higher-voltage motor is known; however, high-power up-converters are very expensive.
Thus, a system which facilitates the use of a higher voltage for vehicle cranking, if desired, while not requiring a voltage up-converter will prove cost-advantageous.